The communication industry is experiencing unprecedented growth resulting from increased use of electronics for sharing voice, data, and video information from any point in the world to any other point in the world. Both the number of people or users wishing to transmit information and the amount and types of information to be transmitted are increasing at exponential rates. As a result of this growth, there is a need for increased bandwidth of the underlying communication systems, e.g., to enable more information exchange to be handled by existing electronic equipment; thus the increased interest in broadband communications.
One part of a communications system that is particularly important to achieving broadband communication capabilities is the signal processing and signal amplification used for signal transmission. To maximize the signal transmission capabilities, the communication signal processing and the amplifiers need to operate as efficiently as possible. One example is the need for efficient broadband signal amplification in wireless communication systems.
In the wireless communications industry, a premium is placed on the ability to amplify wide bandwidth signals, e.g., spread spectrum signals, in a highly efficient manner. As an example, a typical eighteen-channel base station requires approximately 540 watts of RF power output (30 watts per each channel). Assuming a typical power amplifier efficiency of 5 percent, the amount of power needed to generate a RF power output of 540 watts will be 10.8 kW, with 10.26 kW being dissipated as heat. This dissipated heat represents a drawback in that it not only requires the use of fans and heat sinks to cool the base station, but also translates to wasted energy. In short, the cost to operate a wireless base station increases as the efficiency of the power amplifiers used in the base station decreases. Amplifier efficiency is also important in mobile stations that rely heavily on battery power.
Although various attempts have been made to address amplifier efficiency, it remains difficult to design a high efficiency power amplifier that is able to linearly amplify wide bandwidth signals. The amplification of spread spectrum signals, for example, code division multiple access (CDMA) signals, which typically have high peak-to-average signal amplitude ratios, make it impossible to continuously operate a power amplifier in saturation, thereby reducing the efficiency of the power amplifier even further.
One method that has been proposed to improve amplifier efficiency is envelope elimination and restoration (EER). EER is a technique through which highly efficient radio frequency (RF) power amplifiers can be combined to produce a high efficiency linear amplifier system. In this method, a modulated input signal is split into two paths: an amplitude signal path through which the envelope of the modulated input signal is processed, and a phase signal path through which the phase modulated carrier of the modulated input signal is processed. The envelope of the modulated input signal is amplified through an efficient amplifier, which produces an amplified envelope signal. A high frequency amplifier is then used to modulate the high frequency phase modulated carrier with the amplified envelope signal, thereby generating an amplified replica of the original modulated input signal. The amplifier that generates the amplified envelope signal acts as the power supply to the high frequency amplifier.
Although the use of an EER amplifier system to amplify wide bandwidth modulated signals is, in general, beneficial, its efficiency and maximum modulation bandwidth need be further improved to support signals with even greater bandwidth and increased signal traffic.